It is well known that the whiteness and thereby the attractiveness of coated papers can be improved by the addition of optical brighteners and shading dyes to the coating composition.
WO 0218705 A1 however teaches that the use of shading dyes, while having a positive effect on whiteness, has a negative impact on brightness. The solution to this problem is to add additional optical brightener, the advantage claimed in WO 0218705 A1 being characterized by the use of a mixture comprising at least one direct dye (exemplified by C.I. Direct Violet 35) or pigment (exemplified by C.I. Pigment Violet 23) and at least one optical brightener.
In order to satisfy the demand for coated papers of higher whiteness and brightness, there is a need for more efficient shading compositions.
Surprisingly, we have now discovered certain shading dyes which have a strongly positive effect on whiteness while having little or no effect on brightness, and which can be used in coating compositions comprising optical brighteners, white pigments, primary binders, and optionally secondary binders in order to enable the papermaker to reach high levels of whiteness and brightness.
Therefore, the goal of the present invention is to provide aqueous coated compositions containing derivatives of diaminostilbene optical brightener, certain shading dyes, white pigments, primary binders, and optionally secondary binders, which afford enhanced high whiteness levels while avoiding the disadvantages characterized by the use of shading dyes (loss of brightness) or pigments (lower whiteness build) recognized as being state-of-the-art.